Inclusion Criteria:

          -  Confirmed pathologic diagnosis of triple negative breast cancer, OR Prior diagnosis of
             ER or P-R positive breast cancer [HER2 negative] that is demonstrated to be both ER
             and P-R negative (no or rare staining) on the patient's most recent biopsy.

          -  Patients with measurable or non-measurable metastatic disease (RECIST 1.1).

          -  At least one prior treatment for metastatic disease.

          -  Availability of adequate tumor tissue for exploratory analysis and plan to obtain the
             material.

          -  Patients must have recovered from the acute toxic effects of all prior chemotherapy,
             immunotherapy, or radiotherapy prior to entering this study. No chemotherapy or
             radiotherapy may be given within 2 weeks prior to the start of protocol treatment.

          -  Patients must be ≥ 18 and < 80 years old.

          -  Performance Status: Eastern Cooperative Oncology Group (ECOG) 0-2.

          -  Life expectancy of greater than 12 weeks.

          -  Patients must have recovered from uncontrolled intercurrent illness including, but not
             limited to, ongoing or active infection, symptomatic congestive heart failure,
             unstable angina pectoris or cardiac arrhythmia.

          -  Required Laboratory Values: Absolute neutrophil count (ANC) ≥1,250/mm3, platelets
             ≥75,000/mm3, hemoglobin ≥8.5 g/dL, total bilirubin ≤1.5 x ULN, Aspartate
             Aminotransferase (AST)/Alanine Aminotransferase (ALT) ≤3.0 x ULN, alkaline phosphatase
             ≤2.5 x ULN, Patients must have either a normal serum creatinine (<= IULN) OR estimated
             creatinine clearance ≥ 60 ml/min (Cockcroft-Gault formula) within 14 days prior to
             registration.

          -  Concomitant Medications: Erlotinib is primarily metabolized by CYP3A4. Patients CANNOT
             be receiving enzyme-inducing or enzyme inhibiting agents listed here: Inhibitors:
             Amiodarone, Amprenavir, Atazanavir, Chloramphenicol, Clarithromycin, Conivaptan,
             Cyclosporine, Darunavir, Dasatinib, Delavirdine, Diltiazem, Erythromycin, Fluconazole,
             Fluoxetine, Fluvoxamine, Fosamprenavir, Imatinib, Indinavir, Isoniazid, Itraconazole,
             Ketoconazole, Lapatinib, Miconazole, Nefazodone, Nelfinavir, Posaconazole, Ritonavir,
             Quinupristin, Saquinavir, Tamoxifen, Telithromycin, Troleandomycin, Verapamil,
             Voriconazole. Inducers: Aminoglutethimide, Bexarotene, Bosentan, Carbamazepine,
             Efavirenz, Fosphenytoin, Griseofulvin, Modafinil, Nafcillin, Nevirapine,
             Oxcarbazepine, Phenobarbital, Phenytoin, Primidone, Rifabutin, Rifampin, Rifapentine,
             St. John's wort, Sulfadimidine, Sulfinpyrazone, Troglitazone, Troleandomycin. All
             concomitant medications must be recorded.

          -  Sexually Active Patients: For all sexually active patients, the use of adequate
             contraception (hormonal or barrier method of birth control) will be required prior to
             study entry and for the duration of study participation. The non-pregnant status will
             be determined in all women of childbearing potential.

          -  Patients must have signed an approved informed consent.

        Exclusion Criteria:

          -  Active central nervous system (CNS) disease

             a. Subjects with a history of CNS metastases or cord compression are allowable if they
             have been clinically stable for at least 6 weeks since completion of definitive
             treatment, are off steroids (if the steroids were part of the CNS disease treatment),
             and in the case of brain metastases, have stable or improved imaging at least 6 weeks
             after completion of their definitive treatment.

          -  Any serious medical or psychiatric illness that would prevent either the giving of
             informed consent or the receipt of treatment.

          -  Patients pregnant or nursing.

          -  Patients who have used tobacco or nicotine products or medications within the last
             three months given their significant effect on erlotinib drug levels.

          -  Diabetes. Defined as HgbA1C ≥ 6.5%.

          -  Prior metformin treatment OR EGFR targeted therapy.

          -  Rapidly progressive disease as judged by the investigator (Examples include rapidly
             deteriorating performance status or symptomatic lymphangitic spread).

          -  Patient has any condition associated with increased risk of metformin-associated
             lactic acidosis (e.g. congestive heart failure defined as New York Heart Association
             (NYHA) Class III or IV functional status, history of acidosis of any type; habitual
             intake of 3 or more alcoholic beverages per day).